The invention concerns a gradient coil system for the production of a magnetic transverse gradient field G.sub.x =dB.sub.z /dx in an nuclear magnetic resonance (NFR) tomograph with a main field magnet for the production of a homogeneous static main magnetic field B.sub.z in a measuring volume whose center coincides with an origin of a Cartesian x-, y-, z-coordinate system, wherein the main magnetic field B.sub.z is directed along the z-axis and there exists an axial and a transverse access to the measuring volume, and the magnetic transverse gradient field G.sub.x varies along the x-axis of this coordinate system, wherein the gradient coil system comprises four partial coils (S.sub.1x, S.sub.2x, S.sub.3x, S.sub.4x) each having two current connections (A.sub.1, A.sub.2), which are arranged mirror symmetrically with respect to the xy-plane (z=0) and mirror symmetrically with respect to the zy-plane (x=0), wherein each partial coil contains winding sections on an inner and on an outer cylinder Z.sub.1x, Z.sub.ax extending about the z-axis and, in each case, in a radial connecting plane V.sub.+x, V.sub.-x essentially parallel to the xy-plane, wherein the winding sections of each partial coil have current flowing through them in series during operation and wherein the radial connecting planes V.sub.+x, V.sub.-x of those partial coils (S.sub.1x, S.sub.4x ; S.sub.2x, S.sub.3x) which lie across from each other relative to the xy-plane are separated from another.
A gradient coil system of this kind is, for example, known from U.S. Pat. No. 5,414,360.
Whereas in the past, tomography systems have been used practically exclusively for diagnosis purposes, in the future there is an ever increasing demand for combined systems, where therapeutic measures can instantly be followed and controlled by means of tomography devices. Numerous therapeutic measures, as for example surgical, in particular micro-surgical operations or radiation treatments, require as unobstructed an access to the patient as possible. However, in conventional NMR systems this access is blocked by all three field-generating components, that is by the main field magnet, the gradient coil system as well as by the rf resonator.
With respect to the main field magnet the problem is already solved, for example by a magnet system known from U.S. Pat. No. 5,168,211 with a transverse field coil similar to a Helmholtz coil, which, because of its construction principle, represent particularly little obstruction for the free side access to the measuring volume.
The problem of unobstructed side access through the rf resonator has, for example, been solved in the U.S. Pat. No. 5,414,360 by means of the asymmetric saddle coil described in this reference, whereby the feature of a sufficient homogeneity of the generated rf field, important for a tomograph, is provided for.
In order to enable performance of minimal-invasive methods (so-called "key-hole surgery") where due to the lack of a direct field of view onto the operating zone and because of the sometimes very small operation opening of the patient, NMR monitoring is a decisive help to the operator for on-line observance during the operation, the transverse access to the measuring volume inside the NMR apparatus with a sidewise angle of access being as large as possible, should not possibly be obstructed by the gradient coil system.
For example from U.S. Pat. No. 4,486,711, a gradient coil system in the form of single or multiple saddle coils is known, which face each other pairwise on an azimuthal section about the z-axis. Such a gradient coil system is located in the axial bore of the main field magnet and generally penetrates the axial region around the coordinate center, i.e. just including also a lateral gap which is kept free by the above-mentioned special configuration of the main field magnet and the rf resonator. Since the known transverse gradient coils, in particular the shielding coils generally used in combination with these, have their highest winding density just in the region of the central plane z=0, the advantage of transparency and possibility of patient manipulation due to the special configuration of main field magnet and rf coil system is again lost completely. On the other hand, a spatial arrangement of the known saddle coils, which are only located on cylinder surfaces about the z-axis and with the limiting condition that no coil part protrudes into the gap region, would lead to strong non-linearities of the gradient fields generated, to very weak gradient strengths and to large stray fields in the region of the cryostat, i.e. to the generation of eddy currents during gradient switching, which for their part can disturb the homogeneity of the static magnetic field B.sub.0 in the measuring volume.
From U.S. Pat. No. 5,198,769, tesseral gradient coils for NMR tomography apparatuses are known where partial coils of the gradient coil system face each other symmetrically to the z=0 plane and to a plane perpendicular thereto, for example y=0, which each comprise two azimuthal segments with different radial distances r.sub.1 and r.sub.2 from the z-axis, which each comprise the same z-position. By this configuration, the parasitic magnetic field generated by the gradient coil system with field components perpendicular to the z-axis, which induces currents in the measuring object and in the cryostat of the main field magnet, shall be considerably reduced. With a set of two coils of this kind on both sides (with respect to the xy-plane) of the above-mentioned access gap to the measuring volume of the measuring device, one could theoretically construct a gradient coil system with an unobstructed transverse access to the measuring volume and with an axial bore to accept the patient. However, the linearity of such a coil configuration is severely limited.
In U.S. Pat. No. 5,414,360 mentioned above, an NMR measuring device with a gradient coil system had been improved in that x- and y-gradients can be generated simultaneously and in that the tesseral gradient coils generate on the one hand a magnetic gradient field inside the measuring volume which is as linear as possible and that on the other hand there is little or no obstruction at all of a tesseral or oblique access to the measuring volume, thereby enabling free access to the measuring volume.
This purpose had been achieved in that each partial coil comprises several windings and that both azimuthal segments exhibit an axial distance from each other in the direction of the z-axis, wherein the radially outer segment with the radial distance r.sub.2 from the z-axis is with respect to the z-axis axially closer to the coordinate center than the radially inner segment with the radial distance r.sub.1 from the z-axis, and wherein both segments are connected by conductor segments and are both commonly located on a rotational symmetric or ellipsoidal surface r(z). Such a configuration can generate a tesseral gradient field whose linearity inside the measuring volume is comparable to that of tesseral gradient fields generated by classical saddle coils or streamline coils, whereby the side access to the measuring volume is not obstructed by the gradient coil system and with the further advantage of a parasitic contribution of radial field components of the gradient field produced outside the measuring volume which is small.
Because of the arrangement of all conductor elements of, for example, an x-gradient coil on a rotationally symmetric surface r.sub.x (z), all partial coils of this gradient coil can, for example, be mounted on the surface of a nearby mounting surface r.sub.x1. The surface of all components of the completed x-gradient coil then again represents a similar and nearby surface, at which the partial coils of the y-gradient coils--each one rotated by 90.degree. with respect to the corresponding partial coils of the x-gradient coil- can be attached and fixed. By means of field calculations one could show that such coil systems lead to quite satisfactory imaging performance.
However, in the conventional gradient coil systems for tomography systems with side access, the inductance for given gradient strength per unit current is quite large. In addition, the system is not shielded to the outside, or only by additional shielding coils, possibly leading to considerable eddy current problems during gradient switching.
In order to shield the outwardly directed effect of the gradient coils, in many known systems active shielding coils for each partial coil of the gradient coil system are provided for, which comprise a larger radial distance from the z-axis than the gradient coils themselves. For example from U.S. Pat. No. 5,323,135, a transverse gradient coil system to generate an x-gradient G.sub.x is known which, apart from the four partial coils for the generation of the x-gradient G.sub.x, comprise four further partial coils for shielding the gradient coils. Altogether, the known x-gradient coil system comprises eight partial coils, of which the actual gradient coils are located on an inner cylinder, and the shielding coils on an outer cylinder about the z-axis.
A disadvantage of these known gradient coil system .is that only the partial coil regions which are positioned in the vicinity of the xy-plane (z=0) are helpful in generating the transverse gradient. The return sections of the partial coils are in contrast useless or even destructive with respect to the linearity of the transverse gradient to be generated. In addition, these return sections add to the electrical resistance as well as to the total inductance and to the total length of the gradient coil system.
In this respect, the gradient coil system according to GB 22 65 986 A represents an improvement. Per gradient direction, the system presented there comprises only four partial coils instead of eight, whereby each coil comprises two cylindrical sections and a planar section in a plane perpendicular to the z-axis, connecting both cylindrical sections. In contrast to the configuration of U.S. Pat. No. 5,323,135, the return arcs are, in a cartain sense, guided in a plane radially outwards and close on a shielding cylinder with larger radius. Thereby, the connecting plane V of the sections of each partial coil is always on the part of the coil facing away from the xy-plane. It is therefore at maximum distance from the central plane.
The gradient coil system according to GB 22 65 986 A comprises smaller resistance, a smaller inductance and a smaller axial extension along the z-axis than, for example, the system described in U.S. Pat. No. 5,323,135. Moreover, this gradient coil system yields transverse gradients with better linearity and no "gradient reversal" is observed in the region of the return arcs.
In GB 22 65 986 A only configurations are described where the shielding windings located on the outer cylinder extend from the radial connecting surface V to the central plane (z=0). The connecting surfaces V of both cylinders, where the radial sections of the respective partial coil windings are located, has therefore maximum distance from the xy-plane. In general, such a configuration does not perfectly shield the effect of the transverse gradient towards the outside, since in the region of the respective connecting surface V unshielded stray fields remain which can only be compensated by currents which, seen from the central plane (z=0), would have to flow on the other side of the connecting surface V. With the gradient coil configuration according to GB 22 65 986 A, this is, however, not possible.
In this conventional gradient system according to GB 33 65 986 A, there is no remaining transverse access.
It is therefore the purpose of the present invention to present a gradient coil system of the above mentioned kind, which in addition to a small inductance also comprises a good efficiency as well as good shielding.